Technical Field
The present invention relates generally to wireless communications, and more particularly, but not by way of limitation, to systems and methods for frequency synchronization of Doppler-shifted subcarriers.
History of Related Art
A wireless communications system may use an Orthogonal Frequency Division Multiplexing (“OFDM”) transmission scheme. In a wireless communication system that employs an OFDM transmission scheme, a digital data stream is divided into multiple substreams and each substream is sent in parallel over a different subcarrier frequency. The subcarrier frequencies are chosen such that the subcarriers are orthogonal to one another. Because the data is transmitted over multiple frequencies, systems that employ an OFDM transmission scheme (“OFDM systems”) may generally be considered to be in the category of multicarrier systems, as opposed to single-carrier systems, in which data is transmitted on one carrier frequency.
One advantage of OFDM is the ability to efficiently transmit data over frequency-selective channels by employing Inverse Fast Fourier transform (“IFFT”) and Fast Fourier transform (“FFT”) algorithms. OFDM receivers can use digital signal processors and a relatively simple channel-equalization method (e.g., a one-tap multiplier for each tone) as opposed to more complex equalizers typically used by single-carrier receivers. The orthogonality of OFDM subcarriers also increases spectral efficiency and enhances resilience to multipath fading. However, despite such advantages, OFDM systems may be more timing-offset sensitive than single-carrier systems. Also, demodulation of a signal with carrier-frequency offset can cause a high bit-error rate and may degrade performance of a symbol synchronizer in an OFDM system.